The use of -glucans in complementary and alternative medicine (CAM) has prevailed for centuries in traditional Asian medicine. However, their clinical use against malignancies in Western medicine has experienced an oscillating success due to a lack of understanding in their exact mechanisms of action. We have previously demonstrated that soluble yeast -glucan of low molecular weight is capable of binding to the lectin site of complement receptor 3 (CR3, CD11b/CD18) and priming CR3+ effector cells for cytotoxicity of iC3b-opsonized tumor cells. The work presented here shows that particulate yeast -glucan has more unique features than soluble -glucan. The objective of the proposed study is to elucidate the cellular and molecular mechanisms of action for particulate -glucan in tumor immunotherapy. Preliminary studies demonstrated that particulate -glucan, not soluble -glucan, stimulated macrophages and dendritic cells (DCs) for activation and proinflammatory cytokine production. Furthermore, orally administered particulate -glucan stimulated DCs that captured apoptotic tumor cells for activation and maturation, leading to the enhanced anti-tumor CD4 and CD8 T cell responses. Strikingly, oral particulate -glucan treatment altered the tumor microenvironment toward Th1 response. Particulate -glucan was also shown to activate the complement system via an alternative pathway, leading to C5a deposition on tumors. These preliminary data provide a strong basis for the current study. Aim 1 will dissect which signaling pathway(s) may be critical for particulate -glucan-mediated DC activation and cytokine release. Toll-like receptor pathway MyD-88- and TRIF-deficient mice, CR3-deficient mice, and dectin-1 pathway dectin-1- and CARD9-deficient mice will be used to address this issue. In addition, T cell differentiation (Th1, Th2, and Th17) in the presence of particulate yeast -glucan stimulated DCs will be examined both in vitro and in vivo. Aim 2 will test the hypothesis that particulate -glucan modulates suppressive cells existing in the tumor microenvironment thereby promoting Th1 response. The function of myeloid-derived suppressive cells (MDSCs), dynamic interaction between regulatory T cells (Treg) and Th17 cells, cross-talk between MDSCs and tumor-infiltrating macrophages will be examined. Lewis lung carcinoma transfected with surrogate antigen ovalbumin and tumor-associated antigen MUC1 will be used as the model systems to address those issues. Aim 3 will test the hypothesis that particulate -glucan stimulated complement activation may be partially contributed to the enhanced anti-tumor T cell responses. Furthermore, the requirement for C3a and/or C5a and their receptors, C3aR and C5aR, as modulators of anti-tumor T cell responses will be determined. Further knowledge of the mechanisms involved in these effects of particulate -glucan will be of great value in the design of effective clinical applications. Public Health Relevance: This study will elucidate the cellular and molecular mechanisms of action of particulate yeast -glucan in tumor immunotherapy. The data generated in this study will allow for the rational design of immunotherapeutic protocols usable in clinical trials.